Zeolites are crystalline aluminosilicate microporous compositions that are well known in the art. There are over 150 species of both naturally occurring and synthetic zeolites. In general, the crystalline zeolites are formed from corner-sharing AlO.sub.2 and SiO.sub.2 tetrahedra and are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent crystal structure.
Other crystalline microporous compositions are known which are not zeolitic (non-zeolitic molecular sieves or NZMS), but which exhibit the ion-exchange and/or adsorption characteristics of the zeolites. These include: 1) crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440; 2) silicon substituted aluminophosphates as disclosed in U.S. Pat. No. 4,440,871; 3) metal substituted aluminophosphates as disclosed in U.S. Pat. No. 4,853,197; 4) metal sulfide molecular sieves disclosed in U.S. Pat. No. 4,880,761 and 5) metallo zinc-phosphate compositions disclosed in U.S. Pat. No. 5,302,362.
Molecular sieves (both zeolitic and NZMS) are usually hydrothermally synthesized from a reaction mixture in a batch reactor. A continuous process would have the advantage of reduced capital investment, space requirement, operating costs, consistent quality, greater efficiency and waste handling. There are several references which teach continuous processes for the synthesis of zeolites. For example, EP 021675-B1 teaches using a continuous-stream process to prepare zeolites. The process involves preparing a reaction mixture and passing the mixture through a heated reaction zone.
U.S. Pat. No. 3,425,800 discloses a process for the continuous preparation of zeolites comprising forming a zeolitic gel and then supplying the gel to a stratified zone where the zeolite crystals form and settle downward into a lower stratum where they are collected.
U.S. Pat. No. 3,866,094 discloses a continuous process for preparing a hydrocarbon conversion catalyst which involves as one step continuously preparing a zeolite from a reaction mixture containing seed particles. U.S. Pat. No. 5,427,765 discloses preparing zeolite beta by first continuously forming granular amorphous aluminosilicate and then crystallizing the zeolite beta. Lastly, U.S. Pat. No. 5,100,636 discloses preparing zeolites by sending a reaction mixture through a pipe segment for an initial crystallization and then to an open vessel for a second crystallization.
In contrast to this art, applicants have developed a process to not only continuously prepare molecular sieves with high efficiency, but to also control the particle size and the particle size distribution. For example in the case of aluminosilicate zeolites, the process involves continuously adding reactive sources of aluminum, silicon, and at least one structure directing agent into a continuous crystallization reactor having at least two stages. The mixture thus formed is flowed through the reactor in order to crystallize the zeolite. Finally, an effective amount of interstage backmixing is introduced in order to control the particle size and particle size distribution. The greater the amount of backmixing (deviation from plug flow), the larger the particle size and the wider the particle size distribution. Alternatively, the approach to plug flow can be controlled by the number of stages in the reactor. The greater the number of stages, the closer the approach to 100% plug flow, i.e., batch performance.
In a specific embodiment of the invention, the molecular sieve product stream from the continuous reactor is split into at least two streams, each stream is continuously wet milled (in parallel) with different severity and then reblended. In this manner, one obtains at least a bimodal distribution. To applicants' knowledge, there is no mention in the art of ways to both continuously synthesize molecular sieves, control particle size and control particle size distribution.